CA2247135C

CA2247135C - Electrochemical treatment of water contaminated with nitrogenous compounds

Info

Publication number: CA2247135C
Application number: CA002247135A
Authority: CA
Inventors: Mei Lin; Gene Sidney Shelp; Ward Chesworth
Original assignee: Enpar Technologies Inc
Current assignee: Enpar Technologies Inc
Priority date: 1996-02-22
Filing date: 1997-02-21
Publication date: 2006-08-08
Anticipated expiration: 2017-02-21
Also published as: CA2247135A1

Abstract

Waters contaminated by nitrogenous compounds such as nitrate, ammonium, etc., are treated by electrochemical transformation of the contaminant to nitrogen gas. Electrodes are placed in the contaminated water to form a cell, in which the voltage of one of the electrodes is set to the Eh voltage at which nitrogen gas is thermodynamically favoured.
The cell may be electrolytic or galvanic.

Description

1 Title: ELECTROCHEMICAL TREATMENT OF WATER CONTAMINATED WITH

z NITROGENOUS COMPOUNDS

4 This invention relates to the treatment ofi polluted or contaminated water by a electrochemical treatment (which includes both galvanic and electrolytic treatment).

~ The invention is intended to be applied to waters contaminated by dissolved nitrogenous compounds.

s 9 in conventional cases where electrolysis has been used for treating contaminated 1 o water, the reactions that have been utilised have been those, for example, which take 1 ~ metals out of solution by cathode deposition.

13 Work has been done on the use of electrolysis for treating effiluent from manufacturing 14 establishments, where the water is polluted with a specific pollutant. But those 1 s systems were special in that the effiluent water was already contained, and the ~ s research effort was directed to cleaning up or recovering specific contaminants, at 17 known concentrations, and in a system that was designed to cater for the pollutant 1 a when the factory was built.

2o Ammonium (NH4+) in sewage water can be broken down by bacterial action. Much 21 consideration and effort has been applied over many years to engineering the zz ammonium breakdown reaction on a large scale, in the context of municipal sewage z3 water treatment. Generally, bacteria promote oxidation of the ammonium into nitrate;

z4 other bacteria then promote denitrification or reduction of the nitrate to nitrogen gas.

25 The reaction has been engineered by providing an environment in which bacteria can z6 effect the oxidation and reduction.

is Some shortcomings of this conventional procedure will now be discussed. The 29 biological oxidation of ammonium to nitrate and the subsequent reduction of the so nitrate to nitrogen gas, has undesirable side-effects. Other compounds of nitrogen 31 are fiormed, such as nitric oxide (NO), nitrous oxide (N20), and nitrogen dioxide (N02), 3z which are considered to be either directly toxic to humans and other animals, or 33 harmful to the ozone layer or other aspects of the environment.

2 1 One problem is that the conversion of ammonium to nitrogen, via nitrate, done z microbiologically, as in the conventional systems, has been far from complete.

3 Significant quantities of the nitrogen oxides can be released, as gases, during the reactions, in addition to the nitrogen gas. When, as in the conventional systems, the s oxidation of ammonium /ammonia occurred biologically (nitrification), the reaction s pathway may be described as:
7 NH4+ / NH3 ___~ N02 ___~ NO (N20) ___> N03 a whereby the harmful intermediate gaseous compounds may escape into the s atmosphere.
11 Aiso, the nitrate produced by the ammonium breakdown cannot be left in the water.
12 And nitrate itself, for example from agricultural run-off, may be present as a 13 nitrogenous contaminant per se, in groundwater.

1 5 When the reduction of nitrate occurred biologically (denitrification), the reaction 1 6 pathway may be described as:
17 N03 ___~ NO2_ ___~ NOX ___~ NZO ___~ N2 1 a Again, the intermediate gaseous compounds may escape.

zo Another point that should be noted is that the efficiency of the conventional 21 biochemical nitrification and denitrification reactions is affected by cold weather, zz whereby it is found that the gases released to the atmosphere in the winter contain an z3 even larger proportion of N20 and NOX gases. This is a marked disadvantage of the z4 conventional systems. In some cases, also, the temperature can be so low that z~ biological reactions substantially do not take place at a11, and breakdown of the zs contaminants has to await warmer weather.

za It is recognised that the electrochemical reactions as described herein have the zs potential to proceed at lower temperatures than the conventional biological processes.
3 ~ They may therefore be suitable for cold-climate applications, where biological 31 remediation is ineffective for most of the year. ' 3z WO 97f3094i PCT/CA97/OOi22 1 It is recognised in the invention that, in order to remove nitrogenous contamination, z the nitrogenous contaminants can be transformed, by engineered electrochemical s processes, directly into nitrogen gas. (Nitrogen gas of course already comprises 4/5 ° 4 of the atmosphere, and its release is not harmful.) s ' s It is also recognised that inorganic electrochemical reactions can be used to drive both kinds of breakdown reaction, i.e the oxidation of e.g ammonium, and the a reduction of e.g nitrate, both to gaseous nitrogen directly, whereby the production and 9 release of the harmful intermediate gaseous compounds is eliminated or reduced.
1 1 Electro-chemical treatment of water containing nitrogenous contaminants has been 1 2 proposed (Lin & Wu, 1995 Jrnl. Env. Sci. & Health, A30, i 445-1456). What is not 13 present in the prior art is the recognition that nitrogenous contaminants can be 14 transformed, by commercially practicable electrochemical processes, directly into 1 s nitrogen gas, and thereby eliminated from the system.

In assessing whether a particular reaction will proceed electrotytically, the reaction 1 s may be compared to the Nernst equilibrium equation:
~ s E = Eo - (RT/nF)loge(Q) zo in which E is the cell voltage, E° is the cell voltage of a standard cell (calculated, or z1 derived from tables), Q is the concentration quotient, being the ratio of the za mathematical product of the concentrations of all the reaction products, divided by the zs mathematical product of the concentrations of all the reactants. For example, at 25°C, z4 the term (RT/nF)fog°(Q) can be evaluated as (0.0592/n)loglo(Q).
zs One aspect of the present invention lies in determining, from an assessment of the z7 Nernst equation, whether electrolysis will be effective in a particular situation, to cause is contaminants to break down, and if so to what, and under what treatment conditions.

sa The Nernsi relationships for many different transformations and conditions may be ° 31 plotted on a predominance field diagram, or phase diagram, or Pourbaix diagram, i.e 3z a nitrogen-oxygen-hydrogen Eh-pH solution phase diagram. Such a diagram is 33 published and available on a chemistry text book basis, as it relates to aqueous s4 nitrogenous compounds. Sets of the Nernst relationships appropriate to the

4 1 nitrogenous compounds are also available in table form, again on a text-book basis, z and that may also be used.

In carrying out a preferred form of the invention, the designer of the system may use s the Nernst relationships in a phase-diagram form, for example. The designer enters s on the phase diagram the Eh and pH that are measured in the contaminated system. ' 7 He then notes, from the diagram, the voltage V-Ngas at which, at the measured pH, s the predominating form of nitrogen is nitrogen gas. A computation is made as to the s voltage difference between V-Ngas and the voltage V-Eh as actually measured.
11 The designer then provides an electrochemical cell, having two electrodes (i.e an 1 z anode and a cathode). If the nitrogenous contaminant is one (e.g dissolved nitrate) 1 s which requires the addition of electrons in order to be transformed into gaseous 14 nitrogen, then it can pick up the electrons it needs from the cathode, and the designer 1 s should arrange the cell so that the water flows by the cathode. The cell should be so 1 s arranged as to its size, and the velocity of movement of water therethrough, that the 17 residence time of the water near the cathode is long enough for the transformation 1 s reaction to be substantially completed.

zo Similarly, if the nitrogenous contaminant is one (e.g dissolved ammonium) which z 1 requires the subtraction of electrons in order to be transformed into gaseous nitrogen, 22 then it can shed the excess electrons to the anode, and the designer should arrange z3 the cell so that the water flows by the anode. Again, the cell should be so arranged z4 a5 to its size, and the velocity of movement of water therethrough, that the residence 25 time of the water near the anode is long enough for the transformation reaction to be z6 substantially completed.

zs In either case, the engineer should set the voltage V-cell which will be applied z9 between the electrodes to a value at which the Eh voltage as measured in the vicinity so of one of the electrodes, after the electro-chemical reaction has been initiated, lies at s 1 a voltage level, V-Ngas, at which nitrogen gas predominates. That is to say, the ~
3z engineer adjusts the electrical energy available at the electrodes to the effect that the 33 Eh voltage near one of the electrodes changes from V-Eh as measured initially to a s4 voltage that lies within the range of Eh voltages at which nitrogen gas predominates.

2 In either case, it may be noted, the other electrode basically serves no fiunction in the s transformation reaction. (On the other hand, as will be explained later, sometimes 4 both the anode and the cathode of the same cell can be instrumental in releasing s gaseous nitrogen.) a 6 It should be noted that in cases where very reductive conditions prevail, the Eh a voltage, V-Eh, as measured, may be negative.

1 o As an example, in a particular case, a body of water contaminated with dissolved 11 ammonium may be identifiied as having an Eh voltage of, say, -0.53 volts. The pH of 12 the body may be measured at, say, 4.5. Given that the pH is not to be changed, the 1 s designer traces up the diagram, at a constant pH, until he comes to an Eh voltage at 14 which the predominant phase of the nitrogen is nitrogen gas.
It is found that nitrogen 1 s gas predominates over a range of Eh voltages at that pH, i.e from about 0.0 volts to 1 s about +0.3 volts. Given that range, the designer preferably should aim for an Eh 17 voltage of +0.15 volts.

a s (It is assumed that the pH of the water is a given, and that nothing is to be done to zo change the pH. Sometimes, however, it is possible economically to change the pH, 21 and in that case such changes can be factored into the computations -- but it is a 22 benefit of the system as described herein that usually the pH
may be left at whatever 2s value obtains naturally, and only the Eh voltage need be manipulated.) 25 In the exemplary case of water contaminated by ammonium at a pH of 4.5, the zs designer should aim to set up the cell so that the Eh voltage of the water in the region 27 of the anode of the cell is about +0.15 volts. The engineer then adjusts the electro-2s chemical parameters (including the voltage applied between the electrodes) in order z9 to produce the required Eh voltage V-Ngas near the anode --being a voltage between so the limits V-Ngas-upper (0.3 volts) and V-Ngas-lower (0.0 volts).
Preferably, the cell 31 is engineered to provide an Eh voltage in the vicinity of the anode of +0.15 volts.

s2 33 To achieve this, the Eh voltage near the cathode might go even more negative, i.e a4 further away firom promoting the gaseous phase of the nitrogen. But that does not s i matter: when the water passes near the anode, nitrogen gas will bubble off, and when 2 the water passes the cathode, nitrogen gas will not bubble off. Only one of the s electrodes -- the anode in this case -- is effective.

Preferably, it should be arranged that the water moves through the cell (or cells) in s such a manner that all the contaminated water is caused to be close to the anode for ' 7 an adequate residence time.
s s It will be understood that when the contaminant is nitrate, the cathode now becomes i o the electrode which is effective to transform the nitrogen into nitrogen gas. In an 1 ~ exemplary case, the Eh might be measured at +1.1 volts, and the pH at 9Ø
Now, 12 having carried out the Nernst calculations (or having inspected the phase diagram) the i s engineer knows to arrange the electrochemical characteristics of the cell so that the Eh voltage as measured in the vicinity of the cathode of the cell lies at about -0.1 volts, since that is a voltage at the middle of the range which, at a pH of 9.0, nitrogen ~ s gas is the predominant phase.
The cathode and anode of the cell might be physically the same. In fact, the polarity ~ s of the cell may be reversed periodically or cyclically. This can be useful for ensuring 2o that all the water gets treated, and also, switching the electrodes might serve to 21 prevent a build up of a coating on the anode, which sometimes is a problem.
22 ' 23 In some cases, the Nernst equation indicates that the transformation to the nitrogen z4 gas phase can take place galvanically, i.e without the input of electrical energy from z5 an outside source, if the cell is engineered appropriately. In that case, the electrodes 2s would be of different materials, the anode being the material that is the more active in 27 the electro-chemical series.
as 29 It should be noted that the change of phase between the nitrogen gas phase and the 3o contaminant phase is gradual, not sudden. Thus, generally, the release of nitrogen, although at a maximum at one particular Eh (for a given pH), still occurs, in most sz cases, over quite a wide range of Eh. Of course, the expert knows that it is not 33 possible to measure Eh voltages of contaminated waters to a high degree of s4 consistency and accuracy, nor is it possible to set the cell voltage all that accurately.

WO 9?!3(3943 PCT/CA97/OU~22 1 It is recognised herein, however, that the margins of Eh voltage between which an z effective release of nitrogen gas will take place are wide enough that the inaccuracies 3 of measurement and adjustment, which are inevitable in practice, can be accommodated.
s The designer must see to it that sufficient electrical energy is available in the cell to keep the Eh voltage in the contaminated water at the desired level, i.e between the a calculated /indicated limits. The resistance of the water should not be expected to 9 remain constant as treatment takes place, and in fact generally the resistance of the 1 o cell may be expected to increase as the contaminant is eliminated.
Thus, in order to 11 maintain the required Eh voltage in the water, more energy must be passed through 1 z the cell. There may come a point, of course, at which it is no longer practical to 13 maintain voltage between the electrodes: but it is recognised herein that, in general, waters that are contaminated with nitrogenous contaminants are of such a conductivity 1 s that the required voltages at the electrodes can be maintained while supplying 1 s electrical energy in an economical manner.

1 a In many cases, it is economical to provide automatic control of the voltages, which 1 s can be useful in a case where the Eh, pH, concentration of the contaminants, etc zo might be subject to variation over a period of time.
z1 zz It may be noted that carbonates, sulphates, chlorides, etc, and the like, may all be 23 present in addition to the nitrogenous contaminants in the water, and all these z4 substances can have an effect on the Eh voltage. However, it is recognised that the zs single measurement of Eh takes care of all the substances. By contrast, it would be zs difficult to determine what the Eh voltage should be, by calculation, taking account of z7 the various substances in their various concentrations. But, in most cases, a single zs measure of the actual value of the Eh of the water is all that is needed:
the voltage 29 that must be obtained to transform the nitrogen to nitrogen gas can also be easily so determined, e.g by inspection from the nitrogen phase diagram. This makes the s ~ system easy to automate: even though the composition of the contaminants may 3z change, as the water passes by, and as a result of the treatment, all that need be ss done to maintain efficiency is to periodically check the Eh and the pH, and adjust the cell voltage accordingly.

s 4 By way of further explanation of the invention, exemplary embodiments of the s invention will now be described with reference to the accompanying drawings, in s which: .

s Fig 1 is an Eh-pH diagram, which indicates the phases of nitrogen under the various 9 conditions;
1 o Fig 2 is a schematic diagram of a municipal sewage treatment installation, which 11 embodies the invention;
12 Fig 3 is a cross-section of an area of contaminated ground around a well, undergoing 1 s electrolytic treatment;
14 Fig 4 is a section corresponding to Fig 3, showing galvanic treatment;
1 5 Fig 5 is a cross-section of an area of contaminated ground, undergoing electrolytic ~ s treatment;
17 Fig 6 is a plan view of an area of ground, undergoing treatment of the kind as shown 18 in Fig 5.

2o The systems shown in the accompanying drawings and described below are examples 21 which embody the invention. It should be noted that the scope of the invention is zz defined by the accompanying claims, and not necessarily by specific features of zs exemplary embodiments.

25 Fig 1 is an Eh-pH diagram for aqueous nitrogen species under the conditions stated.
2s Whether the predominant form of the nitrogen in the water is ammonium, ammonia, 27 nitrate, nitrite, or nitrogen gas, may be derived, according to the diagram, by entering za the actual conditions of the variables, and reading off the redox voltages.
Since it is z9 desired that nitrogen gas be the predominant form, the engineer may read off the so redox voltages, for a given pH, between which N2 gas will predominate.

sz For example, at a pH of 7 the predominance field of nitrogen gas occurs between 33 -250 mV (moderately reducing) and +150 mV (moderately oxidising) at a nitrogen s4 activity of 0.8 atm, oxygen gas activity at 0.2 atm, and the activity of aqueous nitrogen WO 9?!30941 PCT/CA97/OOi22 1 species fixed at 10-3 moles/litre.

s (For the purposes of this specification, the term "activity" is used to indicate the ideal 4 thermodynamic concentration of a chemical species present in the system under s consideration. If the species is present as a gas, the activity may be measured as a s partial pressure; if the species is present as a non-volatile component, it may be 7 measured in terms of a concentration value, for example moles/litre.) s s In applying the invention to the treatment of sewage water, an electrolytic/
galvanic 1 o cell is set up, in which the sewage water is the electrolyte. The purpose of the cell is 11 to enhance the electrochemical oxidation of ammonium in the sewage water, and the 12 reduction of any residual or generated nitrate in the sewage water, both directly to 13 nitrogen gas, thereby minimising the escape of the toxic nitrogenous gases, and 14 allowing the preferential escape of non-toxic nitrogen gas.
~ s Preferably, a bank of electrochemical cells is provided, each cell having anodic and 1 ~ cathodic compartments connected by a salt bridge or an ion-selective membrane.
1a 1 s Fig 2 is an example of a water treatment plant. Raw sewage enters the primary zo clarifier 20, where initial settling of solid material takes place. The water then passes 21 to a cell 23. The cell is divided by an ion-selective membrane 25, the two zz compartments of the cell being given anode 27 and cathode 29 status by the action of 23 a voltage source 30.

2s Having passed through the anode 27, in which the ammonium in the sewage water is z6 oxidised and removed, the water passes to the aerobic tank 32, where BOD
27 (Biological Oxygen Demand) is reduced. (The tank 32 is of conventional design, 2a being, for example, a trickling tank, or an activated sludge tank.) Sludge is separated 2s and removed from the water in a settling tank 34. Biomass recycling takes place due 3o to the interaction of the tanks 32 and 34.

32 Water from the settling tank 34, which contains nitrate, passes to the cathode 33 compartment 29 of the cell 23, where the nitrate is broken down and removed. As s4 shown, the water from the settling tank 34 may first be passed through one or more powered cells 36, which serve to ensure that the breakdown and removal of all the 2 ammonium and nitrate is as complete as possible.

4 The clean effluent water can be further treated with chlorine, as at 38, if desired.

5 s The electrons released in the anodic compartment 27 by the oxidation of the ammonium are collected by the anode, and pass via external wiring to the cathode a 29, there to promote the reduction of residual nitrate to N2 gas. (If nitrate is absent, 9 under anaerobic conditions hydrogen (1-I+) wilt probably serve as the electron acceptor, ~ o only minor oxygen being present under the existing redox conditions.) ~ 2 Each cell operates independently of the others and can be isolated when maintenance 13 is required.

1 s So long as the redox voltage is maintained within the indicated limits (Fig i ) only a 1 s single-stage treatment system is required. This treatment cell is arranged to control 17 the redox potential of the solution within the predominance field of nitrogen gas.
1s ~ s Two reactions of interest are:
z1 2 NHa+ - N2(gas) + 8 H+ + 6 e-23 2 N03 + 12 H' + 10 e- - N2(gas) + 6 H20 zs Nitrogen gas is the most stable phase under typical earth surface conditions (pH 3-10, 2s redox potential -0.5 to +1.2 volts), so the conversion to gas is nearly always 27 thermodynamically favoured. It is recognised herein that, with the nitrogenous 2$ contaminants, the conditions needed to transform the contaminant to nitrogen gas are 2s within comparatively easy reach, from which it is recognised that the size of the facility 3o needed to treat the water, residence times, etc, can be engineered on an economical 3 ~ scale.

s3 As to their physical structure, the electrodes of the cell 23 may be of porous graphite, 34 iron, magnetite, etc. The set-up generally requires separate anodic and catholic WO 9713094? PCTlCA97/00122 ~i 1 compartments, connected via an ion-selective membrane.
z s The anodic reactions may be described as follows. More than 90% of the nitrogen in ° 4 sewage water is present as ammonium (NHa+), and it is the aim to oxidise the s ammonium directly to nitrogen gas. The reaction to be encouraged is:
' s 2NH4+ - N2 + 8H+ + 6e-7 The standard cell voltage, E° in the Nernst equation, typically would be -0.28 V.
a 9 The electrolytic cell is used to provide the electrical energy required to initiate this otherwise unfavourable chemical reaction.

12 The Nernst equation is utilized to evaluate the optimal extent of the electrolytic 13 conversion. The Nernst relationship shows how redox potential of the half-cell 14 changes with concentrations of reactants and products:
1 s E = -0.28V + 0.0788 pH - 0.0098 !og PN2 + 0.0197 log NH4+
1 s from which it can be seen that E (voltage) becomes more positive by (a) maximising 1 ~ the pH, (b) maximising the NH4+, and (c) minimising the p(N2) by withdrawing the 1 a . nitrogen gas as it is produced.

ao However, the small coefficients on the various terms in the Nernst equation indicate z1 that the potential should be affected only very slightly by changes in the pH and az ammonium content of the influent solution, and by the pressure of nitrogen (in 23 practice, fixed to a value near the atmospheric level of 0.78 atm).

is The nitrate is broken down in the cathode compartment 29. The cathodic reactions is may be described as follows. The intended cathodic reaction is:
a~ 2 N03 + 12 H' + 10 e- - N2(gas) + 6 H20 zs The standard cell voltage, E°, would be 1.24 V, whereby this is a spontaneous as reaction. The reactions that result in the formation of Nz0 and NO are also 3o spontaneous; however, by controlling the voltage within the field of predominance of 31 N2 gas, as described, the formation of the nitrous and nitric oxide species may be 3a inhibited.

34 The Nernst relationship for the above equation is:

W~ 97/30941 PCT/CA97/00122 1 E = 1.24 V - 0.0709 pH - 0.0059 log PN2 + 0.0118 fog N03_ z a It is an aim of the system as described that ammonia /ammonium in sewage water 4 may be broken down, by electrolysis, directly into nitrogen gas. This is contrasted ' s with the conventional biochemical reaction, which has many intermediate stages that s can lead to the release of toxic gases. ' a The invention may be applied in applications other than municipal-scale sewage s treatment, as will now be described.
The invention may be used to treat groundwater contaminated with nitrate. The 1 z cathodic reaction is as described previously; but the anodic reaction will probably 13 involve the oxidation of water, as follows:
2 H20 = 02 + 4H+ + 4e- Eo = -1.23 V
i s Generally, treatment of groundwater contaminated with nitrate has been considered 17 very expensive. It is recognised that major cost benefits arise from carrying out ~ a electrolysis on groundwater in-situ, as compared with electrolysis treatment systems 1 s which involve passing the water to be treated through an engineered treatment facility.
zo Such systems involve the expense of the provision of a means for routing the water to z1 be treated out of the ground, and into and through the facility, and because the facility za itself has to be physically large to contain the volumes of water that need to be z3 treated.

In-situ electrolytic treatment of groundwater incurs reduced expense, especially as to zs capital cost, because the treatment is carried out with the water remaining in the z7 ground, and therefore the cost of engineering a treatment facility, and of engineering a zs means for moving the water into and out of the facility, is avoided or reduced.

so Fig 3 shows a drinking-water well 40, having a metal (steel) casing 43.
Nitrate, e.g 31 from agricultural run-off, is contaminating the local groundwater, to the extent of sz polluting the water drawn up from the well. Carbon rods 45 are inserted into the 33 ground in such a way as to make contact with the nitrate-contaminated water. A
s4 voltage source 47 is connected as shown, so as to turn the well-casing 43 into a WO 99!30943 PCTlCA97100l22 1 cathode and the rods 45 into an anode.

3 The influence of the cathode at the well extends several meters into the surrounding ground, and of course is strongest close to the well. As described, the cathodic s reaction serves to reduce or eliminate the nitrate from the water.
' 6 7 Sometimes, even a galvanic cell (i.e a cell having no source of electrical energy) can a create a sufficiently vigorous reduction of the nitrate that the nitrate is treated s effectively. Fig 4 shows the anode 49 in such a case. The anode is of an electro-1 o active metal, such as magnesium.

1 z Fig 5 shows a case where nitrate-laden water is to be treated, not at a well as in Figs 13 3 and 4, but while passing through the ground. Two carbon rods 50,52 and a voltage 14 source 54 are so placed that the cathode 50 lies within the area of contaminated 1 s groundwater, and the anode 52 lies in the groundwater outside the contaminated 1 s area. (The demarcations between contaminated and uncontaminated of course are 1 ~ characterised by gradual, not sharp, changes in concentration of the contaminant.) 1 s As shown in Fig 5, sensors 56 are provided for monitoring the pH and Eh of the ao groundwater. Signals from these sensors are processed by a computer 58, and the z1 result fed to the voltage source 54. Thus, if the Eh (or indeed the pH) of the water za should change, the voltage is adjusted. Similarly, a check can be kept that the right i3 amount of electrical energy is being supplied to keep the voltage at the desired level.
24 The "before" and "after" measurements can be compared, for checking the efficacy of is the treatment system.
2s a7 Sometimes, only one cell is needed. If more than one is needed, the extra cells are as added in the same manner, ie with the cathode inside the contaminated area and the as anode outside. Fig 6 shows a plan of a typical area of ground in which the 3o groundwater is contaminated with nitrate. Two cells are shown, and other cells may 31 be added later, as the contaminated zone moves, or to give more efficacy to the 32 treatment.

34 In Fig 6, two cells are provided. The required voltage is supplied between the 1 cathode 60 and the anode 62 of the first cell 63. The cathode 60 creates an area of z influence 64, in which the nitrate is transformed into nitrogen gas. The anode has s substantially no effect on the contaminant. The second cell 67 is similar.

s In Fig 6, as shown, the reason treatment is being undertaken is because drinking-s water wells 68 are located in the path of the oncoming plume of nitrate.
These wells 7 may be further protected from any nitrate that escaped the cells 63,67. The protection a is the same as shown in Fig 3.
s 1 o When more cells are added, these can be added as electrically-separate units, or they 11 may be connected in series, or in parallel, with the existing cells. The engineer 1 z should make sure the voltage at the electrodes are maintained, and the manner of 1 s connection is secondary to that.

15 The invention is intended for use generally in setting up large scale electrolysis cells, 1 s preferably in-situ, with or without added electrical energy. Electrodes may be 17 engineered to be suitable for the needs of the individual case.

1 s The engineer should take account of the effects of electrochemical activity at the ao anode (in both the electrolytic and the galvanic cases) in that electrolysis of the water a1 at the anode will lead to the generation of hydrogen ions, and an increase in acidity.

zs 1f the water is naturally alkaline, a little extra acidity would not matter. However, if the a4 water is naturally acidic, further acidity might be a problem. If so, the engineer might 25 decide to provide the anode in a sacrifical material, whereby acidity generated at the is anode would be reduced.
z7 as Since the material of the sacrifical anode passes into the water, the material should 29 be selected on the basis of being environmentally friendly in water.
Magnesium, for so example, has very low toxicity in water, and is a good choice for the sacrifical anode.
s1 Aluminum, on the other hand, can be toxic in water, and is contra-indicated. In some 3a cases, for example if the water contains such potentially toxic materials as dissolved ss aluminum and other metals, the preferred sacrificial anodic material would be iron, s4 because iron can promote co-precipitation of dissolved metals.

WO 9713094f PCTlCA97/OOI22 1 Other areas of applicability for the technology are:
z - Treatment of well water. Many wells are contaminated with nitrate. The electrolytic s celi in the well converts nitrate to nitrogen gas.
4 - Treatment of an aquifer contaminated with nitrate, at depth (where there is little 5 biological activity to reduce the nitrate). The electrolytic cell arises by placing ° s electrodes in the aquifer. Electrolysis reduces the nitrate to nitrogen gas.
7 - Treatment of animal waste collection tanks. Here, the waste material is, for s example, in a tank under a pig barn. in addition to addressing the ammonium, as s described, electrolysis converts hydrogen sulphide to H* and S04 and converts 1 o methane to H* and C02.
- Treatment of mine waste water. Ammonium nitrate + diesel fuel, as used for 1 z explosives, leaves ammonia (NH3) dissolved in the water. Electrolysis converts the 1 s ammonia to nitrogen gas.
- Treatment of waste water from the food processing industry (e.g pickling plants).
1 s Other points to be considered in relation to the treatment of contaminated water by 1 ~ electrolysis as described herein are:
1 a - The efficacy of the electrolysis reaction is generally much less dependent on 19 temperature than biochemical reactions. Biological denitrification at 10°C may be zo , expected to be an order of magnitude less than at 25°C. The electrolysis reaction can z 1 be expected to take place efficiently even in prolonged freezing weather.
22 - The conventional biological treatment produces sludge. The electrolysis reaction z3 can be expected to produce a comparatively smaller amount of sludge.
z~ - Electrolysis circumvents the need to add a carbon source, which is needed in z5 conventional biological denitrification for maintaining reaction rate and completion.
zs This may also reduce the volume of sludge produced.
z7 - Biochemical reactions typically proceed slowly, whereby long residence times are za required for treatment to be completed. The electrolysis reactions can be expected to zs be completed in shorter periods, thus avoiding the tong residence times.
so - If an electrolysis system is found to be inadequate, often the inadequacy can be s1 remedied simply by adding a further electrolytic treatment facility, in series with the 3z already-present facility. The expense of doing that is hardly more than the expense of ss providing the larger system originally would have been. By contrast, if a biological 34 system is found to be inadequate, generally it cannot be remedied in that way, i.e 1 simply by adding another small system. Rather, what is required is that the a inadequate biochemical system must be removed, and a whole larger system installed 3 in its place. Therefore, the prudent designer ofi a conventional biochemical system had to take care to make a contingency provision, at the time the system was installed, often at considerable expense, for future increased demands on the system.

6 The designer of the electrolytic system, on the other hand, can engineer the system '

7 just for today's needs, knowing that the system can easily be upgraded later if the s need should arise.

1 o The term Eh as used herein is defined as follows. The Eh voltage of a solution is the 11 redox potential generated in the solution by comparison with a standard hydrogen ~ a electrode. A standard hydrogen electrode comprises a platinum wire with hydrogen 1 3 bubbling around it, contained within a solution of hydrogen ions in solution of 10°
14 moles per litre (the zero pH condition).

Claims

CLAIM 1. Procedure for in-situ treatment of groundwater contaminated by a nitrogenous contaminant, wherein:
the water is groundwater in its native aquifer in the ground, and the treatment procedure is carried out in-situ, the contaminated water remaining in the aquifer during the procedure;
the procedure includes the following steps:
measuring the pH of the nitrogen-contaminated groundwater;
determining, for that pH as measured, the voltages V-Ngas-upper and V-Ngas-lower, being the limiting Eh voltages between which the predominant nitrogen phase is nitrogen gas;
providing a pair of electrodes in the nitrogen-contaminated-water, in the ground;
so arranging the electrodes in the ground, that the electrodes are in electrolytic communication with each other through the nitrogen-contaminated-water, in the ground;
thereby creating an electrochemical cell in the water, in the ground, being either an electrolytic or a galvanic cell, the nitrogen-contaminated-water comprising the electrolyte of the cell;
measuring the Eh voltage V-Eh of the nitrogen-contaminated-groundwater;
controlling the voltage V-cell, being the voltage as measured between the in-ground electrodes of the in-ground cell, to such a value as to maintain an Eh voltage in the water in the vicinity of one of the electrodes at a voltage V-Ngas, being an Eh voltage between V-Ngas-upper and V-Ngas-lower;
whereby the nitrogenous contaminant in the groundwater is transformed, in-situ, in the ground, into nitrogen gas;
providing means enabling the resulting nitrogen gas to bubble up out of the water and out of the ground;
the Eh voltage of a solution being the redox voltage generated in the solution by comparison with a standard hydrogen electrode.
CLAIM 2. Procedure of claim 1, including the step of maintaining the Eh voltage of the contaminated water substantially halfway between the said voltages V-Ngas-upper and V-Ngas-lower.
CLAIM 3. Procedure of claim 1 including the step of providing a nitrogen-oxygen-hydrogen Eh-pH phase diagram, and of using same to determine the V-Ngas-upper and V-Ngas-lower voltage levels.
CLAIM 4. Procedure of claim 1 including the step of so arranging the cell that the contaminated water comprising the electrolyte of the cell is water that is moving or flowing relative to the electrodes.
CLAIM 5. Procedure of claim 4, including the steps of establishing the presence of a body of the contaminated water, of providing a plurality of the cells in the body of contaminated water, and of so arranging the cells that the contaminated water flows from cell to cell.
CLAIM 6. Procedure of claim 1, wherein, in a case cohere a desired V-Ngas voltage is determined to be lower than V-Eh, whereby electrons need to be added to the contaminant to transform the nitrogen in the contaminant to the gaseous nitrogen phase, the procedure includes the step of ensuring the water moves or flows past the cathode.
CLAIM 7. Procedure of claim 6, wherein the contaminant in the groundwater is nitrate.
CLAIM 8. Procedure of claim 1, wherein the contaminant in the groundwater is nitrate arising from agricultural run-off.
CLAIM 9. Procedure of claim 1, including the step of so arranging the electrolytic cell that the treatment procedure is carried out at a depth in the ground below that at which biological denitrification takes-place.
CLAIM 14. Procedure of claim 1, including the step of ensuring, by on-site measurement, a sufficient size of the cell and the electrodes, and of ensuring a sufficient residence time of the water in the vicinity of the electrodes, as to ensure effective transformation of the nitrogenous contaminant to nitrogen gas.
CLAIM 11. Procedure of claim 7, wherein the cathode of the cell is a metal casing of a well, and wherein the procedure includes the step of providing the anode in the form of a~
conductive rod inserted in the material of the aquifer, within a few metres of the well.
CLAIM 12. Procedure of claim 1, including the step of providing a voltage control means, which is effective to increase or decrease the electrical energy supplied to the electrodes, for maintaining the voltage V-cell between the electrodes at the said level.
CLAIM 13. Procedure of claim 12, including the steps of:
monitoring the pH and Eh of the contaminated water, and computing a consequently-required level of the voltage V-Ngas, on an on-going basis;
providing a voltage control means, which is responsive to the said consequently-required level of the voltage V-Ngas, and is effective to increase or decrease the electrical energy supplied to the electrodes, for maintaining the voltage V-cell between the electrodes at the consequently-required level.
CLAIM 14. Procedure of claim 1, wherein the material of the anode is sacrificial, in that the material of the anode is of the kind that passes into solution in the water as a result of electrochemical activity.
CLAIM 15. Procedure of claim 14, wherein the contaminated water is acidic.
CLAIM 16. Procedure of claim 14, wherein the material of the anode is, or includes, magnesium.
CLAIM 17. Procedure of claim 14, wherein the contaminated water contains a dissolved metal, and the material of the anode is, or includes, iron.
CLAIM 18. Procedure of claim 1, including the step of applying and maintaining electrical energy from an outside source between the pair of electrodes, of such magnitude as to maintain the said voltage V-cell between the electrodes.
CLAIM 19. Procedure of claim 1, including the step of, by use of the Nernst equation, determining a level V-Ngas of the Eh voltage that will produce a transformation of the contaminant into gaseous nitrogen by that reaction.